a. Field of the Invention
The present invention is concerned with a compound transistor circuitry which is a combination of a non-saturation type field effect transistor, and a saturation type field effect transistor or a bipolar transistor, with or without a reactance element.
B. Description of the Prior Art
A field effect transistor (hereinafter to be referred to briefly as FET) of the prior art has a characteristic resembling that of a pentode vacuum tube, and presents a saturation phenomenon in its drain current-drain voltage characteristic. Although this known FET is different from known bipolar transistors in that the former is of the voltage-controlled type whereas the latter is of the current-controlled type, the known FET has characteristics which are very closely similar to the characteristics of the bipolar transistor. Accordingly, said known FET (which will hereinafter to be referred to simply as the saturation type FET) is used under the operation conditions which are substantially similar to those for the pentode vacuum tube or the bipolar transistor. In other words, the saturation type FET is, usually, operated in the range where its drain current is saturated. In this range, this saturation type FET exhibits a very large internal dynamic resistance.
Recently, however, there has been proposed and placed in the market a non-saturation type FET having non-saturation characteristics closely resembling those of a triode vacuum tube, unlike the saturation type FET. As an example of this non-saturation type FET, there is a vertical type junction FET.
This non-saturation type FET does not develop saturation phenomenon in its drain current-drain voltage characteristic, but presents a very low internal dynamic resistance which is comparable with the triode vacuum tube throughout the entire range of operation. Accordingly, this non-saturation type FET is quite suitable for driving a load of a low impedance. Also, because of its low internal dynamic resistance, this type of FET is hardly affected by its internal electrostatic capacity and by the stray capacity. Thus, this non-saturation type FET is advantageous in carrying out an amplification of a high frequency signal.
This non-saturation type FET, however, having on the one hand the aforesaid advantages has on the other hand a shortcoming that, in case this FET is over-driven, its drain current will exceed the allowable value and accordingly it is then easily damaged. For example, in the event that the pulsive noise signal of a large amplitude is applied to the gate electrode of this FET in operation, there will flow a pulsive current of a very large amplitude through the drain electrode. And, accordingly, the peak value of this current will exceed the allowable value of the drain current of this FET, and as a result, the FET itself could be damaged. In this respect, there would be a need for bringing forth some way to prevent the occurrence of such an accident.
Also, in the aforesaid vertical type junction FET, there will flow a very large current through its drain electrode in case its gate circuit is opened. The value of this drain current is determined by the voltage of the power source which is connected between the drain and the source electrodes and by the impedance of a load which is inserted in series in the path of flow of the drain current. Accordingly, in case an amplifier or the like device for driving a low impedance load is constructed by this non-saturation type FET, the drain current will surpass its allowable value at the time the gate circuit is opened and will cause damage to this FET. However, it is not desirable, from the viewpoint of effectively displaying the advantages of the non-saturation type FET, to augment the impedance of the load for the purpose of preventing such a breakage of the FET.